The invention pertains to conductivity measuring methods and apparatus of the type in which eddy currents are induced into the test part causing a back electro-motive force that produces a measurable change in the impedance of a probe coil.
The use of eddy current probing methods and apparatus to measure the conductivity of metal parts is well known and widely used. For example, the measurement of conductivity in this manner is particularly useful in connection with the quality control testing of structural parts made of alloys, such as those of aluminum, in which excessive deviations in the conductivity of the test part is indicative of an improper mixture of alloyed metals, and/or incorrect heat treatment. Eddy current conductivity testing for defective structures and structural parts is used for example in nondestructive testing of aircraft frame components, especially component structures made of various aluminum alloys.
In the use of known eddy current conductivity testers, the instrument produces an analog or digital signal representing the level of conductivity of the probed part and the operator must correlate such measured data with various reference tables that list acceptable conductivity values for various alloys and their respective temper specifications. The required time and tediousness of this task reduces the rate of part inspection to extremely low productivity levels and are the cause of high operator fatigue and excessive test errors.
Additionally, there is difficulty in keeping track of the number of parts that have been tested by the operator and hence no reliable way of determining whether all of the parts or test locations have been actually inspected. Other shortcomings of available eddy current testers include the lack of accuracy in the test results due to unavoidable, thermal drift of the test apparatus circuitry and the absense of any satisfactory way of ensuring an acceptable degree of calibration of the test instrument during any given batch of tested parts.
Still another difficulty in the use of available eddy current test instruments is the difficulty of reading the conductivity data under adverse probing conditions. The probe which is usually mounted on the end of a test cable, must during certain required inspections, be moved into relatively confined areas in which it is difficult or impossible for the operator to read the output data each time the probe is placed on an inspection point. For example, operator difficulty is encountered in this respect when probing inside an aircraft wing. In such case it is necessary to have an assistant read the instrument and coordinate the test with the probe operator, thereby decreasing the productivity of the inspection process and increasing the likelihood of test errors.
Accordingly, it is an object of the present invention to provide an eddy current test apparatus that overcomes one or more of the above-mentioned difficulties and disadvantages associated with the construction and operation of prior conductivity testers of this kind.